Q&A #18523

Cross multiplying

T2T || FAQ || Ask T2T || Teachers' Lounge || Browse || Search || T2T Associates || About T2T

View entire discussion

From: Margaret <culfran2@tpg.com.au>
To: Teacher2Teacher Service
Date: May 06, 2007 at 23:49:47
Subject: Cross multiplying

We have been working on early algebra.  I am  trying to teach my son
about doing the opposite operation in equations to isolate the variable.

He solves by inspection frequently and can often just see things in one
and even two step equations. He has been doing what I used to call cross
multiplication to solve equations involving fractions or division. He saw
the pattern very quickly himself. I never taught him this, he just worked
out that was how to get the answer.  His view is that he doesn't really
need to understand why it works because he has found a method.

I get a bit nervous because I wonder whether or not his chosen method
(not just in this but in other maths subjects- eg: he multiplies normal
numbers algebraically too) will ever disadvantage him in higher maths.

It feels to me like he is using a different system for each operation. I
try to be as flexible as I can be because I know in maths if you get
something one way and people try to teach you an alternative, it becomes
very frustrating.

I know most kids at schools just learn methods and don't really know or
care why they are doing things that way. So what am I really asking?  I'm
not really sure.  Maybe just seeking some reassurance from the creative
teachers here that it's okay to go with the flow and also that I won't be
disadvantaging him if he only uses cross multiplication, rather than
actually understanding what you are doing is an inverse operation.

Post a public discussion message
Ask Teacher2Teacher a new question

[Privacy Policy] [Terms of Use]

Math Forum Home || The Math Library || Quick Reference || Math Forum Search

Teacher2Teacher - T2T ®
© 1994- The Math Forum at NCTM. All rights reserved.